Eyeballing a gap

When you’re entering a harbour or a narrow bay and you need to give one side a certain amount of clearance, bear in mind that while it may be impossible in practice to be sure of keeping a certain measured distance from a point, the eye can readily divide a picture up into halves or thirds. If the entrance is 600 yards across, you can be fairly confident of estimating 200 yards on the basis that it’s a third of the way across. Half-way is easy. At a pinch, you can even subdivide the halves into quarters.